Increasing perceptual difficulty is associated with activation of frontal cortex using positron emission tomography (PET). Activation of cerebral blood flow (rCBF) during stimulus encoding was seen in young subjects in left frontal cortex and right hippocampus but not in old subjects, suggesting age-related impairment of learning processes. rCBF activation during a working memory task indicates that young subjects use right hemisphere (perceptual) strategies for remembering faces with short delays and switch to more left hemisphere (analytical) strategies with longer delays. Old subjects rely more on posterior cortical networks for face recognition regardless of the delay. A study of auditory selective attention using functional magnetic resonance imaging (fMRI) suggested attention-related enhancement of activation in auditory cortex. Sparing of cognitive function early in Alzheimer's disease (AD) is probably due to compensatory mechanisms through synaptic plasticity or reorganization of functional networks.